A well-recognized goal within the field of solid-state color image sensors is the production of a solid-state color imager which is highly sensitive to light, and which produces a clear image while being inexpensive to manufacture. In pursuit of this goal, a number of different types of solid-state color imagers have been produced.
In one such imager, panchromatic image sensing elements in an array are selectively sensitized to color by means of an integral array of color filters disposed over the array of image sensing elements. Highly efficient configurations for such filter arrays maximize the amount of usable information based on human visual acuity for color detail and have been described for example in U.S. Pat. No. 3,971,065 issued July 20, 1976 to Bayer and U.S. Pat. No. 4,047,203 issued Sept. 6, 1977 to Dillon. However, the resolution capabilities inherent within such arrays are limited by the number of sensing elements that can be placed on the array and are further limited in that only a portion of each element in the array contributes to the resolution of fine detail. Consequently, the spatial resolution of such integral-filter color image sensing arrays, while optimized for the particular design, will not be as high as monochrome image sensing arrays of the same number of elements.
Another structure, proposed within British Pat. No. 2,029,642 and Japanese Patent Applications Nos. 55-39404, 55-277772, 55-277773, and 51-95720, is constructed such that the photosensor is superimposed on top of the information transfer device or solid-state base which is capable of a switching function. The base may be an MOS or CCD switching device. Such devices are described in detail within United Kingdom Pat. No. 2,029,642 the disclosure of which is incorporated herein by reference. Such structures have potentially high sensitivity due to a larger sensing area than is present within conventional imaging devices where the photosensor is located on the same level as the information transfer device. However, such devices must utilize multi-color filters and the loss of resolution is comparable to normal solid-state imagers as discussed above. In addition, to produce such a structure, the color filters must be arranged in a particular pattern on the image sensing element which creates difficulty in the alignment and bonding of the filters making the production of such devices complex and expensive.
Improvements over the structure disclosed in the aforementioned British Patent are described in European Pat. No. 46396 and Japanese Patent Applicalion No. 56-133880. The improved structure in the European Patent uses three MOS elements on the solid-state base for each image sensing element. The photo-carrier generated in a photoconductive layer is applied to the gate of an amplifying MOS, as compared to the drain or source in the British Patent, which in turn is connected to a switching MOS transistor. The third MOS transistor provides a means for resetting the photoelectric conversion elements. Solid-state color imagers with this structure improve resolution and signal-to-noise ratios by preventing picture cells which are not being scanned from providing false signals.
The improved structure in the Japanese Patent Application No. 56-133880 uses Metal Nitride Oxide Silicon Semiconductor (MNOS) field effect transistors for detecting and storing data for photosignals, as electrical signals. Similarly to the European Patent described above, the photosignal is applied to the gate of the transistor. Solid-state color imagers with this structure give rise to a nonvolatile memory effect which results from use of the MNOS.
Both of the devices described in the European Patent and the Japanese Patent Application No. 56-133880 must still use multicolor filters arranged in a particular pattern on the image sensing element.
A technique for eliminating color filters in a vidicon is taught in U.S. Pat. No. 3,617,753 of Kato et al. The vidicon includes a conventional semiconductor layer having a substrate on a plurality of p-n diodes which store electrical signals representing light intensity. An electron beam scans the p-n diodes to provide video read out. By stepping the thickness of the semiconductor substrate through which the light passes to the p-n diodes, different wavelength light impinges on the p-n diodes, depending on the size of the step. In this manner different groups of p-n diodes can store different color light. Alternatively the p-n diodes can be formed at varying depths from the surface, thereby effectively stepping the thickness of the substrate. In another embodiment, solid state scanning can be provided instead of electron beam scanning. There a junction device and a MOS element is provided at each pixel and selective etching of the substrate results in varying distances between the light receiving surface of the semiconductor substrate and the junction device of the pixel. The apparatus disclosed is not planar due to the stepped or cutout arrangement and does not have the advantage provided by systems using photoconductors as the light responsive element.
A solid-state color image sensing array has been developed wherein the potential resolution is equal to that of a monochrome array of the same size. Such a sensing array has a plurality of superimposed channels (e.g., three superimposed channels for a three-color device) wherein each channel has a different spectral response due to differential absorption of light by a semiconductor material. (See Research Disclosure, August 1978, Vol. 172, Disclosure No. 17240 entitled: "Color Responsive CCD Imager" available from Industrial Opportunities, Ltd., Honeywell, Havant, Hampshire P091EF, U.K.) However, extremely complex and expensive processes are necessary to produce such devices due to the necessity of superimposing the three channels. When utilizing the CCD (charge-coupled device), the channels which carry the information signal must be carefully constructed within precise limitations making the construction complicated and expensive. Although it is possible to produce a single channel on a substrate, it is complicated and difficult to superimpose additional channels thereon.
Devices such as those described in Disclosure No. 17240 indicate that it is possible to produce multiple superimposed varied channels in silicon crystal which can act as multi-channel superimposed color-sensing devices. However, in addition to the expense and complication of their manufacture, as mentioned above, the color separation and selectivity of these devices is poor due to the inherent limitations of the materials used. The materials used in making such devices act as CCD channels which must have good single crystalline properties as well as color selective photosensors.
As mentioned above, there exists a need within the field for a solid-state color imager which is highly sensitive to light and which gives sharp, detailed resolution of the image. By utilizing a device wherein the multi-colored filters are superimposed over the image-sensing elements in an array, the resulting image, as described within U.S. Pat. No. 3,971,065, has limited resolution capabilities, limited sensitivity and is complicated and expensive to produce due to the necessity of precisely placing the multi-color filters. Increased sensitivity can be obtained by utilizing a device wherein a photosensor is superimposed on top of the information transfer device, as described within United Kingdom Pat. No. 2,029,642, European Pat. No. 46396 and Japanese Application 56-133880. However, resolution of such devices is still somewhat limited because they require the use of multi-color integral filters which also increases the complexity and expense of their production. By utilizing a device having a sensing array of a plurality of superimposed channels, it is possible to obtain a resolution equal to that of a monochrome array. However, complex, expensive manufacturing techniques must be utilized to superimpose three channels on top of each other.
The present invention utilizes a plurality of photosensitive layers which are superimposed on each other and over the base thereby increasing sensitivity. Furthermore, the invention eliminates the need for multi-color integral filters since each photosensitive layer detects a different color of light. The device has a resolution equal to that of a monochrome array of the same size and can be produced by simple, conventional, inexpensive techiques.